1. Field of the Invention
The present invention relates to energy absorbing devices. More specifically, the present invention relates to an energy absorbing and load limiting device that can be used in a wide variety of applications, such as helicopter, aircraft, and space vehicle seating mechanisms, automotive or aircraft restraint harness attachments, cargo or high-mass item tie-downs, automotive bumper attachments, and collapsible steering columns.
2. Background of the Invention
Most prior art energy absorbing devices use either bending or shearing to absorb energy by converting kinetic energy into work. Also, most prior art energy absorbers use multiple components, which increases the cost and complexity of these devices.
U.S. Pat. No. 4,358,136 discloses an energy absorbing device for use with a vehicular seat belt. This energy absorber requires multiple parts, including a housing and a metal strip. The energy absorption of this device is a function of friction and plastic deformation of the metal strip, but not shearing. Friction loads will vary, as a function of the speed of operation, and may be altered by contaminants such as oil or dirt.
U.S. Pat. No. 5,639,144 discloses an energy absorbing child seat fastener. This device uses a combination of shear and plastic deformation of a plate to provide energy absorption. However, the weight efficiency of the device is limited because of the single shear plane and the limited amount of bending that occurs during operation. Because the shearing and plastic deformation follow a spiral path, the resultant load from the absorber cannot, in practice, be a constant value or follow a tailored profile. Additionally, the absorber only responds effectively to loads that are applied essentially perpendicular to the plate.
The present invention is a simple, low cost energy absorbing shear strip bender that can be configured to provide constant loads, tailored loads, or adjustable loads. As shown in FIGS. 1a and 1b, the energy absorber of the present invention is a single part comprising a shear plate, a shear strip tab integral to the shear plate, guide grooves in the shear plate, a shear strip integral to the shear plate, a means for attaching the shear strip tab to a first object, and a means for attaching the shear plate to a second object.
The invention may be fabricated from a single sheet or plate of ductile material, e.g., aluminum, steel, polyethylene, polypropylene, or composite materials. Two through-cuts are made on one side of the sheet to form a shear strip tab, preferably, but not necessarily, of a rectangular shape. This shear strip tab is bent about 180xc2x0, preferably to produce the smallest possible radius without fracture of the material. The remaining (un-bent) sheet material forms the shear plate.
As used herein in the specification and in the claims, the term plate means any base material from which the invention is formed. The plate could be flat, spherical, cylindrical, or any shape compatible with the shear strip process. Preferably, the shape of the plate is the shape most compatible with the mating parts within the system.
Starting from both sides of the shear strip tab, guide grooves are cut or otherwise formed on the surface of the shear plate, to establish the paths along which the shear strip will bend and tear. These grooves reduce the material thickness, making the structure weaker than the remaining portions of the shear plate. The material thus shears along the grooves longitudinally, forming an increasing tab length that plastically deforms. In addition, although grooves are described in this specification and the claims for illustration purposes, it should be understood that other forms of structural weakening, e.g., perforations, would suffice to establish the paths along which the shear strip bends and tears.
The present invention absorbs energy by shearing the shear strip tab from the shear plate, along the guide grooves and bending it in the direction of the applied force. To create this action, two opposing forces (one is in reaction to the other) must be independently applied to the shear plate and shear strip tab. Thus, the shear plate attaches to a first object (such as a seat bucket) while the shear strip tab attaches to a second object (such as a structural component of a vehicle), wherein there would be relative motion between the first and second objects (in the event of a crash). As the objects pull in different directions, the load on the energy absorber increases until it reaches a predetermined load limit whereupon the shear strip begins to shear and bend away from the shear plate. The shearing and bending of the shear strip results in a controlled load during displacement, separating the two objects. This displacement is also referred to as xe2x80x9cstroke.xe2x80x9d The direction of relative motion of the objects is generally parallel to the shearing plate but may vary up to 90xc2x0.
In addition to vehicle seats, the present invention could be used in many other applications. For instance, in an automobile bumper system, the shear plate could be attached to the vehicle structure while the shear strip tab attaches to the bumper itself In a seat belt application, the shear plate could be attached to the seat structure while the shear strip tab attaches to a seat belt fastener. As a final example, for an automobile steering column assembly, the shear plate could be attached to the vehicle structure while the shear strip tab attaches to the steering column. In essence, the present invention is suitable for any application in which the energy displacing two connected objects must be absorbed and the load controlled. As mentioned above, further examples of objects that can be coupled together by the energy absorber include aircraft or other vehicle seating mechanisms, energy absorbing landing gear, automotive or aircraft restraint harness attachments, cargo or high-mass item tie-downs, automotive bumper attachments, and collapsible steering columns.
Many other applications exist. These include, but are not limited to, emergency elevator braking mechanisms, shock snubbers, and earthquake protection attachments that absorb energy from relative motion between structures such as buildings or bridges. There are many applications where loads need to be limited to a predetermined value, where energy needs to be absorbed, or where relative motion between objects is needed without losing structure attachment. Shock load snubbing could be used inside electronic devices to limit the loads and protect sensitive electronic gear from experiencing excessive loads if dropped or impacted by another object.
The load limit of the energy absorber is the sum of the force required to shear the shear strip and the force required to bend the shear strip. The predetermined load of the energy absorber can be controlled by selecting parameters such as the material shear strength, the material tensile strength, the material thickness, the material modulus, the shear guide groove depth relative to the base material thickness, the shear guide groove placement, the distance between shear guide grooves, the direction of loading and the bend radius. Using combinations of these parameters and selectively changing the parameters as the system strokes, the present invention can be implemented as an energy absorber with a constant load/displacement profile, a tailored load/displacement profile, or an adjustable load/displacement profile, as desired. It is also possible to provide a tailored or adjustable load/displacement profile by selectively engaging multiple shear strips.
In a preferred embodiment of the present invention, a constant load energy absorber is formed using two substantially parallel shear guide grooves of uniform depth. With the forces applied parallel to the surface of the shear plate of a given thickness, the constant load limit is a function of the guide groove depth, the width of the shear strip (the distance between the guide grooves), and the structural properties of the energy absorber material.
In another preferred embodiment of the present invention, a tailored load energy absorber is formed by varying the material thickness of the shear plate and shear strip, the depth of the guide grooves, direction of the applied load, and/or the width of the shear strip. Changing these variables either increases or decreases the force required to shear or bend the shear strip, to facilitate the tailoring of the energy absorber for specific varying load patterns.
In another preferred embodiment of the present invention, the load/displacement profile of the energy absorber is adjustable, such that a user can select from a predetermined series of loads. The adjustable load energy absorber features multiple shear strip tabs, each providing a specific load limit. By attaching to one or more of the shear strip tabs, the user can adjust the load limit to suit the particular application for which the energy absorber is being used. The more shear strip tabs that are attached, the higher the load limit.
As an alternative to multiple shear strip tabs, an adjustable load energy absorber could use a dial or other mechanical device to set a cutting or scoring blade to produce a specific guide groove depth. Adjusting the blade to furnish a deeper cut would lower the load limit of the energy absorber.
Accordingly, an object of the present invention is to provide a simple, low cost, tailored load (including constant load) energy absorber that is free from the difficulties and disadvantages found in the prior art.
Another object of the present invention is to provide a simple energy absorber requiring only one primary component.
Another object of the present invention is to provide high efficiency load limiting through a combination of plastic deformation and shearing action.
Another object of the present invention is to provide an energy absorber that absorbs energy applied in any direction ranging from parallel to the shear plate to perpendicular to the shear plate.
These and other objects of the present invention are described in greater depth in the detailed description of the invention, the appended drawings, and the attached claims.